1. Field of the Invention
This invention relates to a solar power generation apparatus and a power control device therefor.
2. Description of Related Art
Since the output of a solar cell fluctuates significantly depending upon such factors as the amount of solar radiation, temperature and operating-point voltage, it is desired that the load as seen from the solar cell be adjusted so as to extract the maximum power at all times. To accomplish this, maximum power point tracking control (hereinafter "MPPT control") has been proposed. To realize MPPT control, the operating point of a solar cell array is made to fluctuate by varying the output voltage and current of the solar cell array, which is constituted by a plurality of solar cells, and the output power characteristic of the solar cell array is examined by checking the output power of each operating point. Then, on the basis of the voltage-power characteristic obtained, operating point at or in the vicinity of maximum power of the solar cell array is tracked.
For example, there is an tracking method, described in the specification of Japanese Patent Publication (KOKOKU) No. 63-57807, which utilizes a voltage differential of power, or the so-called hill-climbing method, described in the specification of Japanese Patent Application Laid-Open (KOKAI) No. 62-85312, of searching for the maximum power point in the direction of an increasing change in amount of power.
Further, in art described in the specification of Japanese Patent Application Laid-Open (KOKAI) No. 7-225624, maximal points of power are sensed between the open circuit voltage Voc of a solar cell and the minimum voltage at allowed maximum current, and the operating point of the solar cell is set as that maximal point of power, among those which have been sensed, at which maximum power is obtained.
However, in a case where power generated by the solar cell is small and there is little current, there are instances where the output current value of the solar cell cannot be sensed accurately owing to current sensing accuracy related to the resolution of current detection and due to the effects of noise. In such a case, maximal points of power cannot be acquired and the maximum power point cannot be tracked even if MPPT is carried out.
Ordinarily, the voltage-power characteristic of a solar cell array has the shape shown in FIG. 10A (in which voltage V and power P are plotted along the horizontal and vertical axes, respectively). However, when the output current of the solar cell array is low, a plurality of power maximal points are produced, as shown in FIG. 10B, owing to the effects of current detection accuracy mentioned above. Further, power maximal points of the kind shown in FIG. 10C are produced owing to the effects of noise or partial shade. Partial shade refers to a state in which the light-receiving surface of a solar panel becomes partially shaded for some reason.
When the hill-climbing method, in which the direction of the voltage-power characteristic curve at a peak thereof is found based upon an increase or decrease in amount of change in power caused by varying the operating point, is applied to a case where a plurality of maximal points of power are produced, a maximum operating point (1) can be found and the maximum power Pmax can be extracted from the solar cell array in a case where the initial operating point is in the vicinity of the maximal point (1), which is the maximum power point, shown in FIG. 10B. However, if the initial operating point is a maximal point (2) or is in the vicinity of the maximal point (2), the maximal point (2) or a maximal point (3) is mistakenly determined to be the summit of the "peak" of the voltage-power characteristic curve and, hence, the maximum power Pmax cannot be extracted from the solar cell array.
In particular, when the amount of solar radiation increases in the case of FIG. 10B, the voltage-power characteristic varies in the manner shown in FIG. 10E and there is a tendency for the operating voltage corresponding to the maximum power point to rise.
Though the foregoing has been described with regard to problems of MPPT in the hill-climbing method, the same results are obtained even with MPPT utilizing the voltage differential of power.
By contrast, the method described in the specification of the aforementioned Japanese Patent Application Laid-Open No. 7-22565 makes it possible to set the operating point to that maximal point of power, among the plurality thereof, at which the largest power is obtained. However, the range over which a power converter connected to a solar cell array is capable of operating is limited in the manner illustrated in FIG. 10D. Accordingly, in the case shown in FIG. 10D, the maximum power point (1) cannot be recognized as a maximal point and, hence, the maximum power Pmax cannot be extracted from the solar cell array, even if the voltage-power characteristic curve is traced within the range in which operation is possible.
Further, with the method described in the aforementioned Japanese Patent Application Laid-Open No. 7-225624, scanning is performed over a very wide range from the open circuit voltage Voc of the solar cell to the minimum voltage thereof. Consequently, there is concern that a period of time over which the operating point departs from the maximum power point will lengthen during the time that scanning is being carried out, thereby resulting in waste of the power generated by the solar cell. This means that if scanning is performed over a long interval, the maximum power point will fluctuate constantly owing to a change in amount of solar radiation or a change in ambient temperature. As a result, the maximum power point cannot be tracked precisely and maximum power cannot be extracted.
Further, if the amount of solar radiation fluctuates while the aforementioned broad range is being scanned, the operating points of a different voltage-power characteristic will be sampled and an operating point set based upon these samples will not necessarily be the maximum power point.